In the case of effectively using a flammable gas, it is necessary to separate a gas such as the air from a source gas containing the flammable gas, and concentrate the flammable gas into an appropriate concentration range. Various such apparatuses and methods for concentrating a flammable gas have been proposed, and inventions have been proposed in which a so-called coal mine gas as a flammable gas is used as a source gas, the air (mainly containing nitrogen, oxygen, and carbon dioxide) is separated from the source gas by using an adsorbent, and methane is concentrated for use.
That is, with the use of natural zeolite, which has a very slower adsorption rate for methane than for nitrogen, as an adsorbent (i.e., with the use of an adsorbent that preferentially adsorbs nitrogen, oxygen, and carbon dioxide to methane), a coal mine gas is introduced into an adsorption tower filled with the adsorbent by a compressor or the like until a predetermined pressure is reached. Consequently, oxygen, nitrogen, and carbon dioxide that are contained in the coal mine gas are adsorbed first at the front portion (lower portion) of the adsorption tower, and methane, for which the adsorption rate is slow, is adsorbed at the back portion (upper portion) of the adsorption tower. Furthermore, inventions of apparatuses and methods for concentrating methane have been proposed in which the above-described methane is further released from the upper portion of the adsorption tower until atmospheric pressure is reached.
Thereby, the air can be separated from the coal mine gas as the source gas by using the adsorbent, methane can be concentrated, and the concentrated methane can be used as a fuel or the like. It is thought that the coal mine gas containing a relatively high concentration of methane can be similarly used.
That is, a configuration (hereinafter referred to as “PSA (Pressure Swing Adsorption) device”) is envisaged that includes:
an adsorption tower filled with an adsorbent that adsorbs a miscellaneous gas other than a purification target gas from a source gas;
a source gas supply line for supplying the source gas to the adsorption tower;
a product gas recovery line for discharging the purification target gas that has not been adsorbed on the adsorbent as a product gas; and
a miscellaneous gas discharge line for discharging desorbed miscellaneous gas that has been adsorbed on the adsorbent,
the adsorption tower, the source gas supply line, the product gas recovery line, and the miscellaneous gas discharge line being connected so as to be able to perform a pressure swing operation of alternately performing
an adsorption step of receiving the source gas from the source gas supply line, adsorbing the miscellaneous gas onto the adsorbent, and recovering the product gas, and
a desorption step of desorbing the miscellaneous gas adsorbed on the adsorbent and discharging that miscellaneous gas from the miscellaneous gas discharge line.
Thereby, when the source gas is supplied to the adsorption tower from the source gas supply line, the adsorption step of adsorbing the miscellaneous gas in the source gas onto the adsorbent in the adsorption tower can be performed. The purification target gas in the source gas that has not been adsorbed on the adsorbent is recovered from the product gas recovery line, and the adsorption tower that has been saturated by adsorbing the miscellaneous gas can be regenerated by performing the desorption step of depressurizing-desorbing the miscellaneous gas adsorbed on the adsorbent. The exhaust gas generated at this time is composed mainly of the miscellaneous gas, and is discharged from the miscellaneous gas discharge line. A pressure swing operation of repeating the adsorption step and the desorption step can be performed.
Here, in the case of concentrating methane contained in a biogas or a coal mine gas, an adsorbent having a large adsorption capacity for the miscellaneous gas in the source gas and also having a high methane/miscellaneous gas separation property are desired as the adsorbent. However, not so many adsorbents having a large adsorption capacity for the miscellaneous gas in the source gas and also having a high methane/miscellaneous gas separation property are known. For example, Patent Documents 1 to 5 state that carbon molecular sieves having an average pore diameter of about 3 Å to 5 Å are suitably used for adsorptive separation of methane from various gases (see, e.g., Patent Documents 1 to 5).
However, it has been revealed that, in addition to the pore diameter, the distribution of pore volumes and the adsorption time constant contribute to the miscellaneous gas separation performance (see, e.g., Patent Documents 6 to 9), and there is a need to select an appropriate adsorbent in accordance with the purpose of the pressure swing operation.